I'm familiar with the scenario of Rh- mother with Rh+ fetus having complications (more so after her first child), but that's not what I'm curious about. I want to know mechanistically why a pregnant woman's body does not display a reaction similar to what we see with rejection from a mismatched organ transplant. On the most basic level, I feel as though the fetus should be recognized as "nonself" due to paternal genetic contribution.

I feel as though if this mechanism could be understood, it could be harnessed for other situations (e.g., organ transplants).

Right now I don't remember the full story, but part of it is that pregnant women are somewhat immunosupressed and there is something like a layer of NK cells around the placenta making sure nothing attacks it. Not very practical to adapt to organ transplants.
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jkadlubowskaJan 26 '13 at 8:43

1 Answer
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Just so that someone answers this question so that the moderators have less work to do:

The fundamental reason why embryos don't get massacred by the maternal immune system is because of the placenta and several of its functions. Three mechanisms are:

1) Secretion of neurokinin B, which is also secreted by parasites to avoid detection of the host.
2) The placenta has developed immunoevasive actions from a virus. It creates a syncytium to limit transaction of mobile immune cells.
3)It only allows specific antibodies,(IgG) to enter and bind infection. (IgG being the only isotype here).